Editing Futex (section)

{{Short description|Kernel system call}}
In [[computing]], a '''futex''' (short for "fast userspace [[mutual exclusion|mutex]]") is a [[Kernel (operating system)|kernel]] [[system call]] that [[programmer]]s can use to implement basic [[lock (computer science)|locking]], or as a building block for higher-level locking abstractions such as [[semaphore (programming)|semaphore]]s and [[POSIX]] mutexes or [[condition variable]]s.

A futex consists of a [[kernel (operating system)|kernel-]]space ''wait queue'' that is attached to an [[Atomic operations|atomic]] [[integer]] in [[userspace]]. Multiple [[Process (computing)|processes]] or [[Thread (computer science)|threads]] operate on the integer entirely in userspace (using [[atomic operation]]s to avoid interfering with one another), and only resort to the fast but still more expensive [[system call]]s to request operations on the wait queue (for example to wake up waiting processes, or to put the current process on the wait queue). A properly programmed futex-based lock will not use system calls except when the lock has contention; since most operations do not require arbitration between processes, this will not happen in most cases.